Effect of reduction potential on the rate of reduction of nitroacridines by xanthine oxidase and by dihydro-flavin mononucleotide

Abstract

The cytotoxicity of many nitroaromatic compounds is mediated by bioreduction. There is a correlation between the one-electron-reduction potentials and cytotoxic potencies of simple nitroheterocycles such as nitroimidazoles, but no such correlation is observed with DNA-binding nitroacridines related to the drug nitracrine. A study of the reduction of 1-nitroacridines by the enzyme xanthine oxidase and by reduced flavin mononucleotide, FMNH, (as a model of the enzyme's active site) has been undertaken to explore the redox dependence in this series. The values of V,,,,,/K,,, for reduction by xanthine oxidase, XOD, and the second-order rate constants of reduction by F M N H, , analysed using Marcus electron-transfer theory, indicate a correlation between reduction potential and the rate of reduction, but suggest that the potential-energy barrier for electron transfer is small relative to that imposed by requirements for association, orientation and desolvation of the reactants. The close similarity in Marcus parameters for reduction by FMNH, and by XOD suggests that reduction occurs by a similar mechanism in each case. The free-energy change for electron transfer, AG:, is only 1-2 kJ mot-'. It appears that the rate of reduction is unlikely to be the only determinant of hypoxic cytotoxicity for the nitroacridines, although the applicability of findings with XOD t o other nitroreductases is uncertain.

Quantitative structure activity relationships for the electron transfer reactions of Anabaena PCC 7119 ferredoxin‐NADP+ oxidoreductase with nitrobenzene and nitrobenzimidazolone derivatives: mechanistic implications

“…Correlations reported by Orna et al [18] and O'Connor et al [24] between the logarithm of V max and V max / K m and single or two electron reduction potential values for the reduction of aromatics by other flavoenzymes, such as cytochrome P450 reductase, spinach ferredoxin reductase, or xanthine oxidase, might as well be explained in the framework of the complete ping‐pong kinetic schemes discussed here.…”

“…This can be derived from the fact that the rate equation for this model predicts that k cat = k 2 . Therefore, the observation of a correlation between the natural logarithm of k cat and E(LUMO) of the polynitroaromatic substrates requires the re‐evaluation of the kinetical model often used to describe the oxidation of flavine‐dependent reductases [14,16,18,24,34,35].…”

“…2a). This k cat / K m is a parameter previously often used to define QSARs for the single electron reduction of quinones and nitroaromatics by various flavoenzymes [14,16,18,22,24,34,35]. However, due to the relatively high reactivity and solubility of polynitroaromatic substrates as compared to mononitroaromatic substrates, determination of independent k cat and K m values by Michaelis‐Menten type saturation kinetics became feasible.…”

“…Up to now, attempts to define quantitative structure activity relationships (QSARs) for k cat as well as k cat / K m values for the reduction of various nitroaromatic compounds by flavoenzymes were unsuccessful [14,18,22,24]. Recently, however, we have described a QSAR for the two electron reduction of a series of nitrobenzimidazol(on)es by DT‐diaphorase based on calculated energy of the lowest unoccupied molecular orbital (E(LUMO)) [23].…”

Electron Transfer and Oxidative Stress as Key Factors in the Design of Drugs Selectively Active in Hypoxia.

“…Studies of the rates of reduction of nitroarenes by free, reduced flavin mononucleotide (FMNH 2 ) [79,112] or by xanthine/xanthine oxidase [80,112] in the absence of oxygen have revealed a similar redox dependence. These studies, and earlier work [113] showed that reduction proceeds to the hydroxyamine (four electrons) via the formation of the nitro radical-anion (one electron reduction) [114], which is probably the rate-limiting step.…”